Biomass-derived fuels, i.e., so-called biofuels (e.g., bioethanol, biodiesel etc.) are expected as alternatives to fossil fuels.
Biomass is a raw material for a biofuel, and encompasses oils and fats, saccharides (including starch), or the like. Biomass is produced as a result of photosynthesis of plants. Accordingly, plants that are capable of vigorously photosynthesizing and accumulating oils and fats, or saccharides in their cells can be used as means for producing biomass. Currently, corn and soybean are mainly used for production of biomass. However, corn and soybean are also consumed as food and feed. In consequence, price increment for food and feed due to rapid increase in biofuel production is becoming problems.
In view of the circumstance, biomass production using an alga is attracting attention as biomass production means alternative to corn and soybean (see, for example, Patent Literature 1 and Patent Literature 2). Biomass production using an alga has, for example, advantages of not conflicting with food and feed production and enabling mass propagation.
For example, mutants of Chlamydomonas, which is a kind of alga, such as a cell-wall-deficient mutant and a mutant with a thin cell wall (e.g., cw15, cw92) are known. Such mutants have properties convenient for introduction of DNA into cells from an outside, and are therefore widely used in gene introduction experiments. Further, since cells of such mutants are easy to break, contents in the cells are easy to collect. This leads to increased productivity of biomass. In view of these advantages, studies on biomass production using such mutants are reported. For example, Patent Literature 3 describes a technique of producing oils and fats by using a cell-wall-deficient Chlamydomonas mutant. Non-Patent Literature 1 reports that a Chlamydomonas which suffers from a starch synthesis gene deficiency in addition to the cell wall mutation (cw15) releases oil droplets made of oils and fats out of its cells. Non-Patent Literature 2 reports that productivity of oils and fats is increased by breaking a starch synthesis gene of the cell wall mutant (cw15) of Chlamydomonas. Non-Patent Literature 3 is known as a report about mutation of Chlamydomonas cell walls.
Further, Patent Literature 4 reports a technique of (i) producing starch with the use of chlorella, which is a kind of alga, (ii) releasing the starch out of cells, and (iii) then carrying out ethanol fermentation with the starch.